1. Field of the Invention
The present invention relates to a pneumatic tire for a motorcycle, and more particularly, to a pneumatic tire for a motorcycle in which both wear resistance and wet steering stability can be obtained.
2. Background Art
In order to obtain drainage performance during a motorcycle travel on a wet road surface, grooves are formed on a tread of a pneumatic tire for a motorcycle.
For example, tread patterns shown in FIGS. 5 to 7 can respectively be employed as a tread pattern for a conventional pneumatic tire for a motorcycle.
A tire 100 has tread pattern, shown in FIG. 5, which is a combination of small bock patterns from a viewpoint of drainage performance and a rubber with high elasticity. However, the tire 100 cannot provide effective block rigidity, and has not been suitable for a machine that outputs a large power in respect of wear resistance. In other words, when the tire 100 is used for a large power-outputting machine, edge portions of blocks 102 to 114 at a stamping side and at a kick-out side easily unevenly wear, thus deteriorating steering stability.
A tire 200 having a tread pattern shown in FIG. 6 and a tire 300 having a tread pattern shown in FIG. 7 are respectively designed based on an idea that a grip should be obtained on surfaces of blocks (land portions), as compared with a conventional idea that a grip should be obtained at edges thereof. Since lug grooves substantially extending in a tire transverse direction are not formed in the vicinity of each of the centers of the tires 200 and 300, the respective tires 200 and 300 provide high circumferential direction rigidity and exhibit excellent traction performance and wear resistance. However, from a viewpoint of a ratio of a block pattern to a tire, it can be considered that drainage performance is poor due to a low negative rate, and grip sensation is not granted due to insufficient edge components.
In the tire 200 shown in FIG. 6, circumferential direction linear grooves 204 which extend linearly in a tire circumferential direction are respectively provided at both sides of the tire equatorial plane CL of a tread 202 to define a rib 205 which extends linearly along a tire circumferential direction.
A plurality of inclining grooves 206, which are inclined such that end portions of the inclining grooves 206 at the tire equatorial plane CL side are located closer to a tire rotational direction side than to a tread end side, is formed in a tire circumferential direction at both sides of a pair of the circumferential direction linear grooves 204, and an inclining sub-groove 208 is formed between the inclining groove 206 in the vicinity of the tread end.
The inclining groove 206 at the tire equatorial plane CL with respect to a tire circumferential direction is almost fixed at an angle of 28 degrees, and an angle in the vicinity of the tread end with respect to a tire circumferential direction gradually increases toward the tread end.
The inclining grooves 206 are arranged at a substantially equal pitch in a tire circumferential direction, and terminate in the vicinities of the respective circumferential direction linear grooves 204. As shown by a chain double-dashed line, substantially trapezoidal land portions 210, each of which width at a tire rotational direction side (side in a direction of arrow A) is comparatively narrow, and each of which width in a direction opposite to a tire rotational direction is comparatively large, are continuously formed in a tire circumferential direction.
Arrows marked on the tread 202 of FIG. 6 indicate input directions in which inputs are exerted onto the tread 202 of the tire 200 during a motorcycle travel. During a motorcycle straight drive, portions near the tire equatorial plane CL mainly road-contact, and input directions are directed toward a tire circumferential direction. However, during a motorcycle cornering, input directions are likely to incline with respect to a tire circumferential direction, and the larger the camber angle, the closer the input directions to a tire transverse direction.
By the way, each of the land portions 210 of the tire 200 shown in FIG. 6 has a large difference between rigidity at a tire rotational direction side and rigidity at a side opposite to the tire rotational direction side. Accordingly, a problem is caused in that uneven wear may easily occur due to an input exerted in a tire circumferential direction thereby deteriorating steering stability.
Further, in the tire 300 shown in FIG. 7, three circumferential direction linear grooves 304, which extend linearly in a tire circumferential direction, are provided respectively at both sides of the tire equatorial plane CL of a tread 302. Five ribs 305, which extend linearly in a tire circumferential direction within a comparatively wide range of a tire transverse direction, are formed in the vicinity of the tire center.
Moreover, a plurality of inclining grooves 306, which are inclined such that an end portion of each of the inclining grooves 306 at the tire equatorial plane CL side is positioned closer to a tire rotational direction side than to a tread end, is formed in a tire circumferential direction at both external sides in a tire transverse direction of the circumferential direction linear grooves 304.
The inclining grooves 306 of the tire 300 are inclined at a fixed angle with respect to a tire transverse direction, and arranged at a substantially pitch in a tire circumferential direction.
Accordingly, a plurality of parallelogram shaped land portions 308 is formed in a tire circumferential direction at both external sides in a tire transverse direction of the circumferential direction linear grooves 304.
Arrows marked on the tread 302 of FIG. 7 indicate input directions in which inputs are exerted onto the tread 302 of the tire 300 during a motorcycle travel.
By the way, in the tire 300 shown in FIG. 7, since the ribs 305 which extend linearly along a tire circumferential direction are provided within a comparatively wide range in a tire transverse direction in the vicinity of the tire center, the entire rigidity of the land portions is high with respect to an input exerted in a tire circumferential direction, and it is thereby not concerned that uneven wear may occur. However, since angles of the inclining grooves 306 are fixed, rigidities of the land portions 308 become insufficient with respect to inputs during a motorcycle cornering at a large camber angle, and a problem is caused in that each tread end may easily unevenly wear thereby deteriorating steering stability due to an occurrence of uneven wear.
In view of the aforementioned facts, an object of the present invention is to solve the aforementioned drawbacks, and provide a pneumatic tire for a motorcycle in which both wear resistance and wet steering stability can be obtained.